There are a number of different specific cooling tower configurations for cooling a liquid (e.g., water) stream with a gas (e.g., air) stream. However, such configurations can be characterized as either of the tubular heat exchange type or the direct gas-liquid contact type. In the tubular type, water is passed through tubes formed of conductive material (e.g., copper) which are conventionally provided with fins to present more heat transfer surface to the air. Air flowing across such tubes cools the water sensibly, i.e., without evaporation.
The technique of direct exposure of the water to the air stream is the most common type of cooling tower. Direct contact cooling is significantly more efficient than tubular cooling because the latent heat of evaporation of the water materially assists the cooling process. The cooling tower is filled with medium, conventionally of either the crossflow or counterflow type, formed from relatively inexpensive materials, such as wood or plastic, in comparison to the more expensive heat exchange tubes formed of conductive metal. However, a major disadvantage of evaporative cooling is the loss of a portion of the water to the atmosphere during evaporation. This is a particular disadvantage in areas, such as desert regions, wherein water is becoming increasingly scarce.
Wet cooling is employed in certain cooling and condensing processes, such as steam turbine operations, wherein it is essential to cool the liquid to a low absolute temperature. This is because water can be cooled to a lower temperature in the presence of evaporative cooling. Thus, in the turbine operations, warmer temperatures which would be produced with dry cooling can adversely affect plant cycle efficiencies.
Towers have been constructed which employ dry cooling in a tubular section above a wet cooling section and are designated "wet-dry cooling towers". The partial dry-cooling is utilized to reduce the amount of moisture discharged from a tower where water is in short supply or to minimize an objectionable fog which might be discharged from the tower. It is apparent that in a wet-dry cooling tower, there is a net loss in effective utilization of the air because of the inherent inefficiency of the dry cooling tower. Thus, where the wet-dry cooling tower is employed, as to conserve water, it is important to maximize the efficiency of the wet section of the cooling tower to counter-balance the loss in efficiency of the dry section.
Conventional wet cooling sections are of either the counterflow or crossflow type. The former employs a generally horizontal fill section with an air opening below the lower surface of the same. Counterflow fills of the film type have a relatively good heat transfer coefficient. The air is drawn from below the fill and out the tower by a fan positioned above the fill. When the distance between the fill and base of the tower is relatively small, the air must be drawn from the surroundings into the tower at a relatively high velocity and, when it reaches a position below the fill, it is forced to turn abruptly at a sharp angle to proceed upwardly through the fill. This requires high fan power requirements. On the other hand, by building the tower on relatively high supporting legs, the velocity of the incoming air is somewhat reduced but the overall height of the tower is substantially increased. Among the disadvantages of such height increases are increased pumping head, structural wind loads, and general appearance.
Conventional crossflow cooling sections comprise a relatively thin vertical fill section with the water being fed from an overhead source and the air being drawn therethrough from air inlets at the side of the tower. Since there is no necessity for the air to make radical changes of direction in the fill and the air inlet is spaced along the entire height of the fill, the overall fan power requirements are usually less than those of a conventional counterflow tower as set forth above.
A crossflow cooling section is inherently less efficient with respect to heat transfer than a counterflow section based on a unit of fill. Another disadvantage of a crossflow cooling section is that the water is loaded onto the top of the relatively thin crossflow fill. There is a maximum water load beyond which the water will not redistribute effectively because it will start gushing in a steady stream through the section. When this maximum water load is exceeded in a crossflow section of the film fill type, the water will not cling to the fill leading to relatively poor heat transfer between the air and water. Also, resistance to the transversely flowing air is substantially increased requiring excessive fan power. This problem of water loading cannot be effectively overcome by widening the fill in the direction of air flow because there is a limiting factor on cooling efficiency relative to the thickness of the fill. A major factor in this limit is that the fan power for the longer air path through the fill disproportionately increases in comparison to the advantages to be attained by easing the above water load problems.
A theoretical attempt to combine certain counterflow and crossflow features was made in U.S. Pat. 3,227,429 in which a series of offset cellular units with all walls of the cells inclined to the horizontal are illustrated in FIG. 12. The gas and liquid travel in the same direction, either concurrently or countercurrently, generally parallel to the cell side walls. Since the bottom cell walls or splash plates of the above patent are inclined at a steep angle, the liquid flows through the packing in rushing streams, rather than gradually descending in a gravitating path, and so does not spread out into thin films onto the side walls of the cells. The liquid actually concentrates rather than spreading out. This rapid concentrated flow of liquid greatly reduces the efficiency of the illustrated packing.